Epicyclic Gearboxes

Epicyclic gearboxes or epicyclic gear trains are mechanical devices used both in high-power machines and in small tools.   Thermoplastic resins allow for the creation of small-sized epicyclic gear trains with high performance, reduced weight, and low production costs.
The different configurations of epicyclic gear trains enable transmission ratios ranging from t = 0 to t = 1 (t is the ratio between the output shaft speed and the input shaft speed).
Epicyclic gear trains are divided into simple epicyclic gear trains and compound epicyclic gear trains.

 Simple Epicyclic Gear Train (Example A)

The simple epicyclic gear train is the most common and is generally used for transmission ratios in the range of t = 0,2 – 0,32.
It consists of:

• 1 gear, called sun gear, fixed to the input shaft. The axis of the sun gear is the axis of the gear train.
• 3 or 4 gears, called planet gears.
• 1 fixed ring gear, coaxial with the sun gear.
• 1 rotating element, called planetary carrier, connected to the output shaft. The guide pins for the planet gears are fixed to the planetary carrier.

Operation: The planet gears mesh with the sun gear and the ring gear, and the rotation of the sun gear imparts two movements to the planet gears: one of rotation around their axes and one of revolution around the sun gear axis. Since the guide pins of the planet gears are fixed to the planetary carrier, the revolution of the planet gears causes the planetary carrier to rotate around the axis of the gear train, consequently rotating the output shaft, which is connected to the planetary carrier.
In steel gear trains, designed to transmit high power, the number of planet gears can exceed 4, whereas in thermoplastic resin gear trains, the number of planet gears generally does not exceed 4.
The number of teeth of the sun gear (Z₁), the planet gear (Z₂), the ring gear (Z₃), and the number of planet gears (i) must satisfy the following relationships:

• Z₃ = Z₁ + 2 Z₂
• Z₁ + Z₃ = k i        (k is an integer)

Compound Epicyclic Gear Train

A compound epicyclic gear train is formed by combining two simple gear trains, in which the second gear train uses one or two elements from the first.  Additionally, the planet gears have double teeth. Below are some examples of compound epicyclic gear trains.

Compound Epicyclic Gear Trains for Transmission Ratio t = 0,32 – 1

Compound Epicyclic Gear Train with Fixed Planetary carrier (Example B)

It consists of:

• 1 input sun gear.
• 3 or 4 planet gears with double teeth.
• 1 output sun gear.

In this configuration, the guide pins of the planet gears are fixed to the gearbox, so each planet gear only rotates around its axis. One of the two sets of teeth of the planet gear meshes with the input sun gear, and the second set of teeth meshes with the output sun gear.
This compound epicyclic gear train is advantageous for its simplicity in calculation and the reduced number of components.

Compound Epicyclic Gear Train with Rotating Planetary carrier and Ring Gear Fixed to the Input Side of gearbox (Example C1)

It consists of:

• 1 input sun gear.
• 3 or 4 planet gears with double teeth.
• 1 fixed ring gear inserted on the input side of the gearbox.
• 1 planetary carrier to which the guide pins of the planet gears are fixed.
• 1 output sun gear.

In this configuration, one set of teeth of the planet gear meshes simultaneously with the input sun gear and the fixed ring gear, and the second set of teeth meshes with the output sun gear. Therefore the planet gear, in addition to rotating around its axis, also revolves around the gear train axis.
This compound epicyclic gear train is more complex than the previous one, both in calculation and in the number of components. It is used when it is not possible to fix the guide pins of the planet gears to the outer casing of the gearbox.

Compound Epicyclic Gear Train with Rotating Planetary carrier and Ring Gear Fixed to the Output Side of gearbox (Example C2)

It consists of:

• 1 input sun gear.
• 3 or 4 planet gears with double teeth.
• 1 fixed ring gear inserted on the output side of the gearbox.
• 1 planetary carrier to which the guide pins of the planet gears are fixed.
• 1 output sun gear.

In this configuration, one set of teeth of the planet gear meshes with the input sun gear, and the second set of teeth meshes simultaneously with the fixed ring gear and the output sun gear. Therefore the planet gear, in addition to rotating around its axis, also revolves around the gear train axis.
This compound epicyclic gear train is very similar to the one in Example C1 and is therefore more complex than the one in Example B, both in calculation and in the number of components. It is used when it is not possible to fix the guide pins of the planet gears to the outer casing of the gearbox.

Compound Epicyclic Gear Trains for Transmission Ratio t = 0 – 0,2

Multi-stage planetary gear set

A compound epicyclic gear train consisting of 2 or more identical simple gear trains connected in series, in which the output shaft of the first gear train is fixed to the sun gear of the second gear train.
The advantage of this solution is the possibility of having modular gearboxes with a low investment for the production equipment of the components.
The downside is the greater axial length of this gearbox and the higher total number of components.

Compound Epicyclic Gear Train with Rotating Planetary carrier and 2 ring gears (Example D)

It consists of:

• 1 input sun gear.
• 3 or 4 planet gears with double teeth.
• 1 fixed ring gear inserted on the input side of the gearbox.
• 1 planetary carrier to which the guide pins of the planet gears are fixed.
• 1 rotating ring gear connected to the output shaft.

In this configuration, one set of teeth of the planet gear meshes simultaneously with the input sun gear and the fixed ring gear, and the second set of teeth meshes with the rotating ring gear. In this case too, the planet gear, in addition to rotating around its axis, also revolves around the gear train axis. The rotating ring gear rotates around the gear train axis and is connected to the output shaft.
This gear train allows choosing the transmission ratio in a very wide range, from t = 0,2 to t = 0, and compared to other solutions, it has very small dimensions and a lower number of components. However, the calculation of this gear train is more complex. Furthermore, it is almost always irreversible, meaning that applying torque to the output shaft keeps the input shaft stationary.